1. Field of the Invention
The present invention relates generally to the field of downhole sampling analysis and in particular to storing energy in a storage medium to pressurize a formation fluid sample at down hole pressure and temperature to retrieve the sample to the surface without significant pressure loss on the sample due to a reduction in temperature.
2. Background Information
Earth formation fluids in a hydrocarbon producing well typically contain a mixture of oil, gas, and water. The pressure, temperature and volume of the formation fluids control the phase relation of these constituents. In a subsurface formation, formation fluid often entrains gas within oil when the pressure is above the bubble point pressure. When the pressure on a formation fluid sample is reduced, the entrained or dissolved gaseous compounds separate from the liquid phase sample. The accurate measurement of pressure, temperature, and formation fluid sample composition from a particular well affects the commercial viability for producing fluids available from the well. The measurement data also provides information regarding procedures for maximizing the completion and production of the hydrocarbon reservoir associated with the hydrocarbon producing well.
Downhole fluid sampling is well known in the art. U.S. Pat. No. 6,467,544 to Brown, et al. describes a sample chamber having a slidably disposed piston to define a sample cavity on one side of the piston and a buffer cavity on the other side of the piston. U.S. Pat. No. 5,361,839 to Griffith et al. (1993) discloses a transducer for generating an output representative of fluid sample characteristics downhole in a wellbore. U.S. Pat. No. 5,329,811 to Schultz et al. (1994) discloses an apparatus and method for assessing pressure and volume data for a downhole well fluid sample.
Other techniques enable capture of a formation fluid sample for retrieval to the surface. U.S. Pat. No. 4,583,595 to Czenichow et al. (1986) discloses a piston actuated mechanism for capturing a formation fluid sample. U.S. Pat. No. 4,721,157 to Berzin (1988) discloses a shifting valve sleeve for capturing a formation fluid sample in a chamber. U.S. Pat. No. 4,766,955 to Petermann (1988) discloses a piston engaged with a control valve for capturing a formation fluid sample, and U.S. Pat. No. 4,903,765 to Zunkel (1990) discloses a time-delayed formation fluid sampler. U.S. Pat. No. 5,009,100 to Gruber et al. (1991) discloses a wireline sampler for collecting a formation fluid sample from a selected wellbore depth. U.S. Pat. No. 5,240,072 to Schultz et al. (1993) discloses a multiple sample annulus pressure responsive sampler for permitting formation fluid sample collection at different time and depth intervals, and U.S. Pat. No. 5,322,120 to Be et al. (1994) discloses an electrically actuated hydraulic system for collecting formation fluid samples deep in a wellbore.
Temperatures downhole in a deep wellbore often exceed 300 degrees F. When a hot formation fluid sample is retrieved to the surface at ambient temperature, the resulting drop in temperature causes the formation fluid sample to contract. If the volume of the sample is unchanged, contraction due to temperature reduction substantially reduces the pressure on the sample. A pressure drop in the sample causes undesirable changes in the formation fluid sample characteristics, and can allow phase separation to occur between the formation fluid and gases entrained within the formation fluid sample. Phase separation significantly changes the formation fluid sample characteristics and reduces the ability to properly evaluate the properties of the formation fluid sample.
To overcome this limitation, various techniques have been developed to maintain pressure of the formation fluid sample at a high pressure while retrieving the sample to the surface. U.S. Pat. No. 5,337,822 to Massie et al. (1994) discloses an apparatus that pressurized a formation fluid sample with a hydraulically driven piston powered by a high-pressure gas. Similarly, U.S. Pat. No. 5,662,166 to Shammai (1997) utilizes a pressurized gas to pressurize the formation fluid sample. U.S. Pat. Nos. 5,303,775 (1994) and 5,377,755 (1995) to Michaels et al. disclose a bi-directional, positive displacement pump for increasing the formation fluid sample pressure above the bubble point so that subsequent cooling does not reduce the fluid pressure below the bubble point. These known methods compensate for expected pressure losses on the sample by exerting additional pressure on the formation fluid sample.
The additional pressure is supplied by either a pump or a pressurized nitrogen gas. Thus, the over pressure supplied to the formation fluid sample in the above related sampling techniques is limited by the capacity of the pump or initial pressure of the gas to maintain the sample at single phase conditions (above the bubble point). In some cases, it may be desirable to provide additional pressure on the sample that might exceed the capacity of the sampling pump. Thus there is a need for a method and apparatus that supplies additional pressure on a formation fluid sample that exceeds the pumping capacity of the sampling pump.
The provision of additional pressure from a gas typically requires pumping high pressure fluid or gas into a chamber in a sampling tool at the surface. These pressures can reach 10,000–15,000 pounds per square inch. Such high pressures should be treated with sufficient caution to avoid risk to human life. Thus, there is a need for a gas pressurization system that does not require pumping fluids or gases to high pressures, such as 10,000–15,000 pounds per square inch, at the surface to avoid the risk associated with such high pressures. Typically, the pressurizing modules remain affixed to the sample tank to maintain the sample at or above the in- situ formation pressure at the sampling depth. Thus, there is a need for a removable pressurizing module.